Constant velocity joints are common components in automotive vehicles. Typically, constant velocity joints are employed where transmission of a constant velocity rotary motion is desired or required. Common types of constant velocity joints include plunging tripods, fixed tripods, plunging ball joints and fixed ball joints. These types of joints currently are used in front wheel drive vehicles or rear wheel drive vehicles, and on the propeller shafts found in rear wheel drive, all wheel drive and four wheel drive vehicles. Constant velocity joints are generally grease lubricated for life and sealed with sealing boots when used on drive shafts. Thus, constant velocity joints are sealed in order to retain grease inside the joint while keeping contaminants and foreign matter, such as dirt and water, out of the joint. To achieve this protection, the constant velocity joint is usually enclosed at the open end of the outer race by a sealing boot made of a rubber, thermoplastic, or silicone material. The opposite end of the outer race generally is enclosed by a dome or cap, known as a grease cap. A monoblock or integral stem and race design style does not use a grease cap, but is sealed by the internal geometry of the outer race. Sealing and protection of the constant velocity joint is necessary because contamination of the inner chamber may cause internal damage and destruction of the joint. Furthermore, once the inner chamber of the joint is lubricated, it is lubricated for life.
During operation, the constant velocity joint creates internal pressures in the inner chamber of the joint. The internal pressure is the result of increases in internal or external temperature. These pressures must be vented to the outer atmosphere in order to prevent pressure build-up. If the pressure build-up is allowed to reach a critical state, the boot, which protects the joint from contaminants and water, may deform, crack, deteriorate, or blow out, thus diminishing the life of the boot and losing its ability to properly seal the joint. A constant velocity joint is usually vented by placing a small hole generally in the center of the grease cap or at least one hole around the outer periphery of the boot neck. These methods of venting the pressure build up are sometimes not adequate because if the constant velocity joint is in a static state and not rotating the lubricating grease may settle in the vent hole and block or hinder its venting function and/or evacuate lubricant from the joint. This type of vent may also allow infiltration of contaminants. Once the internal pressure builds up, the joint may fail due to a ruptured boot or other catastrophe. Furthermore, the constant velocity joint, after running for long periods of time, may create very high temperatures along with high pressures which are vented through the current vent holes. However, if the constant velocity joint is submerged or saturated in water or other contaminants, the water will, via a rapid temperature change cause a vacuum, within the joint chamber and draw water into the constant velocity joint, thus contaminating the grease lubricant and reducing the life of the constant velocity joint.
Therefore, there is a need for a constant velocity joint that will prevent the build up of internal pressure while eliminating the possible ingress of contaminants from entering the constant velocity joint.